A significant amount of research has been directed at identifying methods and compositions which, broadly speaking, can be used to stabilize wastes and soils. For instance, numerous methods and compositions have been developed which are capable of stabilizing wastes, particularly contaminated soils, so that the rate at which contaminants leach from the treated waste is minimized. Other methods utilize agents which effectively bind constituents within soil, thus enabling a solid construction material to be prepared from a mixture of the soil and agent. Finally, other methods utilize compositions which have utility in stabilizing soil surfaces, thereby providing a base for construction materials and/or protecting against erosion. Ideally, however, various methods using a similar agent would be capable of achieving any of the foregoing types of stabilization. The present invention provides such methods. In particular, the current invention discloses methods which utilize hexametaphosphate (HMP), with or without other additives such as fly ash, to stabilize waste materials, to prepare construction materials and to stabilize soils against erosion.
Considering waste stabilization first, particularly the stabilization of contaminated soils, methods capable of stabilizing wastes have great utility given society's current concern about properly disposing of waste in order to safeguard human health, wildlife and the environment. This concern is manifested in the numerous state and federal regulations which govern the handling of wastes.
Several general strategies have been developed to meet current regulations or at least partially stabilize wastes. One general approach involves solidification. This approach utilizes binders to produce an end product having low permeability characteristics, thereby significantly reducing the rate at which contaminants leach from the waste. The solidification approach often involves the use of grout, cement, lime and/or silicates as the solidifying agent. Methods using fly ash to encapsulate waste materials fall into this category. Because fly ash, the coal combustion by-product generated in large coal-fired utility plants, has a high silica content, it is possible to initiate a pozzoulanic reaction by pressurizing a mixture of fly ash and water. Waste materials included within a mixture having a high fly ash content can thus be encapsulated (U.S. Pat. Nos. 5,425,807, 5,405,441, 5,374,307 and 5,366,548 to Riddle). Related uses of fly ash are described in U.S. Pat. Nos. 5,154,771 to Wada, et al.; 4,659,385 to Costopoulos, et al.; 3,625,723 to Sicka; 5,534,058 to Strabala; 4,615,809 to King; and 5,627,133 to Nelson.
Chemical stabilization or fixation comprises another general waste treatment method. These methods involve addition of one or more chemical additives to the waste so that contaminants are converted into an insoluble form. The use of phosphate compounds fits within this general approach. Many stabilization methods using phosphate compounds are used in conjunction with a buffering agent or a secondary complexing agent to stabilize heavy metal-containing wastes. Several U.S. Patents are directed at using phosphate compounds solely to treat lead-contaminated materials. Illustrative of this group are U.S. Pat. Nos. 5,193,936 and 5,569,155 to Pal; 5,536,899 to Forrester; 5,545,805 to Chesner; 5,512,702 to Ryan; and 5,5162,600 to Cody.
Other U.S. patents describe the use of phosphates, again generally with a buffering component, to treat wastes containing heavy metals besides just lead. Patents describing this approach include: U.S. Pat. Nos. 5,527,982 and 5,397,478 to Pal; 5,037,479, 5,202,033 and 5,637,355 to Stanforth; 5,591,116 and 5,674,176 to Pierce; and 5,645,518 to Wagh. In some cases these approaches specifically require the formation of a slurry (U.S. Pat. No. 5,645,518 to Wagh) or note that dry mixing of treatment agents may not provide the correct conditions to enable the necessary stabilizing chemical reactions to occur (see for example, U.S. Pat. Nos. 5,037,479 and 5,202,033 to Stanforth). Related methods utilizing phosphate compounds are described in U.S. Pat. Nos. 3,959,975 to Graf; 4,334,029 to Naito, et al., 3,640,021 to Grafmuller; 3,201,268 to Hemwall; 4,919,711 to Banyai, et al.; and 4,231,984 to Hofman.
Turning now to agents which can be used in construction materials, methods of preparing bricks from fly ash have been described (U.S. Pat. Nos. 5,425,807, 5,405,441, 5,374,307 and 5,366,548 to Riddle; 5,154,771 to Wada, et al.; 4,659,385 to Costopoulos, et al.; 3,625,723 to Sicka; 5,534,058 to Strabala; 4,615,809 to King; and 5,627,133 to Nelson). As noted above, this is accomplished by pressurizing mixtures of fly ash and water, preferably in combination with bottom ash, in order to accelerate a pozzoulanic reaction which yields a solid product. Of the soil-based construction materials, adobe bricks are perhaps the most well-known. These bricks are prepared by thoroughly mixing soil and water to produce a plastic form that can be molded into almost any shape.
With regard to compositions which can be used to stabilize soil surfaces, numerous compositions have been developed. These compositions may stabilize the soil surface by actually binding soil particles or by forming a protective film on the soil surface. Ideally, these compositions protect against natural forces and/or against physical impact from foot or vehicular traffic, as well as providing a means for minimizing dust formation.
Many of the compositions which have been developed are complex formulations, often including various organic polymers. A few patents have issued in which the stabilizing agent includes a phosphate agent. For example, U.S. Pat. No. 3,201,268 to Hemwall, describes a method for stabilizing soils by mixing soils having a high clay content with phosphoric acid and water-soluble lead. One disadvantage of this approach is that it involves the addition of lead, a toxic compound, to the soil; the method also requires the use of a very strong acid. Another patent (U.S. Pat. No. 3,640,021 to Grafmuller) describes a mixture including a vinyl polymer and hexametaphosphate.
This brief overview illustrates that although a variety of methods using different agents have been developed to either stabilize wastes, prepare construction materials or stabilize soil surfaces, there remains a need for methods which can use the same agent to accomplish all these tasks. As noted above, the present invention addresses this need by providing methods utilizing hexametaphosphate, a readily available and non-hazardous compound, which provides the desired broad range of soil stabilization. The invention further provides construction materials, soil stabilizing compositions and earthen compositions containing hexametaphosphate.